Production method of aluminum alloy forging for automobile suspension

ABSTRACT

Provided is a production method of an aluminum alloy forging for an automobile suspension having a disturbance affectable surface with not excessively notch-sensitive. The production method includes, as heat treatment processes, a solution heat treatment process, a quenching process, and an artificial age hardening process. The quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to the automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water.

TECHNICAL FIELD

The present invention relates to a production method of an aluminum alloy forging suitable for, e.g., a suspension for supporting a body of a four-wheel automobile.

BACKGROUND OF THE INVENTION

An aluminum 6000 series alloy (Al—Mg—Si series alloy) for a structural member is required to be subjected to a quenching process for obtaining a supersaturated solid solution. Conventionally, a production method has been proposed in which a quenching process is performed by immersing a product in quenching water in a vertical direction or an oblique direction with respect to the water surface of the quenching water to thereby obtain high strength (see Patent Document 1).

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2017-179413

Non-Patent Document 1: Light metals, 1981, Vol. 31, No. 11, 748-757

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, when such a structural member is assembled to an automobile and the automobile actually runs on a road surface, the structural member corrodes due to disturbance, such as, e.g., wastewater and a snow melting agent, from the road surface. The disturbance can occur when rainwater, groundwater, or a snow-melting agent on the road surface comes into contact with a tire to be scattered. Such scattering occurs in the upward direction of the vehicle body from the road surface. The scattered rainwater, etc., positively comes into contact with a surface of a suspension assembled to the automobile body, the surface facing the road surface, which causes a risk of causing corrosion of the suspension. In a situation in which a load is repeatedly applied, such corrosion may result in a starting point of fractures. Therefore, there is a concern that such reliability as a suspension for supporting an automobile body may deteriorate.

Preferred embodiments of the present invention have been made in view of the above-described and/or other problems in the related art. Preferred embodiments of the present invention can significantly improve upon existing methods and/or devices.

SUMMARY OF THE INVENTION

The present invention has been made in view of such a technical background. One of the objects of the present invention is to provide a production method of an aluminum alloy forging capable of suppressing occurrence of stress corrosion cracking due to corrosion by reducing the strength of a surface of the forging where corrosion is likely to occur as compared with a surface where corrosion is less likely to occur to thereby reduce notch sensitivity of the surface of the forging where corrosion is likely to occur.

Note that the above-described theory is based on Non-Patent Document 1. According to Non-Patent Document 1, SCC (stress corrosion cracking) occurs in an alloy in which a protective film, such as, e.g., a passivation film, is easily formed. The protective film is destroyed by shearing of the GP zone by dislocations, resulting in exposure of the metal, which in turn promotes the preferential corrosion reaction. According to Table 3 (described on page 751 in Non-Patent Document 1), the stress corrosion cracking life of the Al-4% Zn-2% Mg alloy subjected to a quenching process in cold water quenching, boiling water quenching, or air cooling is 24 days in the case of the cold water quenching, 62 days in the case of the boiling water quenching, and 365 days in the case of the air cooling, even after the period, no cracking occurs. From this, it can be understood that occurrence of stress corrosion cracking can be suppressed by slowing the quenching rate when performing the quenching process.

The present invention differs in the alloy system disclosed in Non-Patent Document 1. However, the aluminum alloy forging of the present invention is presumed to be equivalent to the alloy system disclosed in Non-Patent Document because the aluminum alloy forging of the present invention is likely to form a protective film, has a strengthening mechanism by a GP-zone, and has a PFZ (precipitate free zone in which precipitates are generated in the vicinity of the grain boundary).

Other objects and advantages of the present invention will be apparent from the following preferred embodiments.

Means for Solving the Problems

In order to achieve the above-described objects, the present invention provides the following means.

[1] A production method of an aluminum alloy forging for an automobile suspension, the method including, as heat treatment processes:

-   -   a solution heat treatment process;     -   a quenching process; and     -   an artificial age hardening process,     -   wherein the quenching process is performed by bringing a lower         surface of the aluminum forging to be disposed on a ground side         when assembled to an automobile into contact with water after an         upper surface of the aluminum forging opposite to the lower         surface is brought into contact with the water.

[2] The production method of an aluminum alloy forging for an automobile suspension as recited in the above-described Item [1],

-   -   wherein an aluminum alloy constituting the aluminum alloy         forging is an Al—Mg—Si based alloy.

[3] The production method of an aluminum alloy forging for an automobile suspension as recited in the above-described Item [1], further including:

-   -   a hot forging process performed immediately before the solution         heat treatment process,     -   wherein the solution heat treatment process is performed in         conjunction with a temperature rise in the hot forging process.

[4] The production method of an aluminum alloy forging for an automobile suspension as recited in the above-described Item [2], further including:

-   -   a hot forging process performed immediately before the solution         heat treatment process,     -   wherein the solution heat treatment process is performed in         conjunction with a temperature rise in a hot forging process.

[5] The production method of an aluminum alloy forging for an automobile suspension as recited in any one of the above-described Items [1] to [4],

-   -   wherein a temperature of the water in the quenching process is         40° C. to 90° C.

Effects of the Invention

In the invention as recited in the above-described Item [1], the quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to the automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water. Therefore, the lower surface of the aluminum forging is less quenched as compared with the upper surface, resulting in lower strength in the lower surface than the upper surface, which in turn results in deterioration in the notch sensitivity of the lower surface. As a result, it is possible to provide an aluminum alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking.

In the invention as recited in the above-described Item [2], it is possible to provide an Al—Mg—Si based alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking.

In the invention as recited in the above-described Items [3] and [4], the solution heat treatment process is performed in conjunction with a temperature rise in a hot forging process. In other words, the solution treatment process uses a temperature rise in a hot forging process. Therefore, it is possible to provide an aluminum alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking at a low cost.

In the invention as recited in the above-described Item [5], the temperature of the water in the quenching process is 40° C. to 90° C. Therefore, it is possible to provide an aluminum alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing the processes in the production method of the present invention.

FIG. 2 is an explanatory view schematically illustrating a quenching process in which a forging is immersed in water at an inclination angle θ with respect to the water surface when immerging the forging in the water.

FIG. 3 is an explanatory view schematically illustrating a quenching process in which a forging is immersed in water in parallel to the water surface when immerging the forging in the water.

FIG. 4 is a perspective view showing a casting used in the production method of the present invention.

FIG. 5 is a perspective view of a forging obtained by the production method of the present invention.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

A production method of an aluminum alloy forging for an automobile suspension according to the present invention will be described in detail. Note that the embodiment described below is merely illustrative, and the present invention is not limited to the embodiment and can be appropriately modified without departing from the technical concept of the present invention.

The production method of an aluminum forging for an automobile suspension according to the embodiment is to produce the aluminum alloy forging 20 for an automobile suspension as shown in FIG. 5 by performing a molten metal forming process, a casting process, a homogenization heat treatment process, a hot forging process, a solution heat treatment process, a quenching process, and an artificial age hardening process in this order (see FIG. 1). Hereinafter, each of these processes will be described.

The molten metal forming process is a process of obtaining an aluminum alloy molten metal prepared by dissolving raw materials and adjusting the composition.

In this embodiment, an Al—Mg—Si based alloy molten metal consisting of Si: 1.00 mass % to 1.20 mass %, Fe: 0.15 mass % to 0.30 mass %, Cu: 0.33 mass % to 0.45 mass %, Mn: 0.48 mass % to 0.54 mass %, Mg: 0.75 mass % to 0.95 mass %, Cr: 0.13 mass % to 0.17 mass %, and the balance being Al and inevitable impurities is obtained.

The casting process is a process of obtaining a casting (forging billet) by casting the aluminum alloy molten metal obtained in the molten metal forming process.

The casting process is not particularly limited, and a conventionally known method may be used. Examples thereof include a continuous casting and rolling method and a semi-continuous casting method (DC casting method), or the like.

The diameter of the casting is not particularly limited, but is set to, for example, 30 mm to 80 mm in diameter. In addition, the casting may be extruded with an extruder to obtain a forging billet. Also in this case, for example, it is set to 30 mm to 80 mm in diameter.

Further, in the casting process, it is preferable to set the cooling rate of the casting to 10° C./min to 50° C./min. By setting so, it is possible to produce an aluminum alloy product having a sufficiently high tensile strength at room temperature. In particular, the cooling rate of the casting is preferably set to 15° C./min to 30° C./min.

The homogenizing heat treatment process is a process in which the casting obtained in the casting process is subjected to a homogenizing heat treatment to homogenize the microsegregation caused by solidification, deposit the supersaturated solid solution elements, and convert the metastable phase to the equilibrium phase.

By performing this homogenization heat treatment, it is possible to make the intermetallic compound smaller, thereby suppressing the destruction starting from the intermetallic compound, which in turn can further improve the tensile strength.

By performing the homogenizing heat treatment, elements contained in the intermetallic compound are uniformly dispersed into the base material, which can further improve the tensile strength by the solid solution strengthening and the precipitation.

It is also preferred to perform the homogenizing heat treatment at the highest possible temperature within the temperature range in which eutectic melting does not occur. By performing under such a condition, the intermetallic compounds are effectively melted and dispersed into the base material. As a result, the intermetallic compounds can be reduced in size.

Further, the treatment temperature in the homogenizing heat treatment is preferably set in the range of 500° C. to 570° C. By performing the heat treatment at a temperature of 500° C. or higher, the intermetallic compounds, such as, e.g., crystallized substances, of the casting can be dissolved and sufficiently homogenized. By performing the heat treatment at a temperature of 570° C. or lower, burning can be prevented.

After performing such a homogenizing heat treatment process, the casting is cut to a predetermined length to obtain a forging billet.

The hot forging process is a process of heating the forging billet obtained after the homogenizing heat treatment process and applying pressure with a press to perform molding.

The temperature condition in the hot forging process is related in that the characteristics of the aluminum alloy are expressed more reproducibly. That is, the microstructure of the aluminum alloy after the solution heat treatment process described later can be made into equiaxed crystal grains. In particular, the hot forging process is preferably performed by setting the mold temperature to 100° C. to 250° C. and the material temperature to 400° C. to 550° C. This is because the tensile strength of the aluminum alloy forging can be further improved by performing the hot forging under such a condition.

Next, the solution heat treatment process, the quenching process, and the artificial age hardening process will be described.

The solution heat treatment process is a heat treatment in which the aluminum alloy forging obtained in the hot forging process is held at high temperature and then rapidly cooled to form a supersaturated solid solution.

In the solution heat treatment process, it is preferable to set the heating temperature to 510° C. to 560° C. and the holding time to 0.5 hours to 6 hours. It is possible to better balance the cost and the characteristics by setting such a condition.

Further, the solution treatment process may be performed in conjunction with a process in which the temperature rise in the hot forging process is used. That is, by performing the hot forging process in conjunction with a process a solution heat treatment, the aluminum alloy forging held at a high temperature immediately after the hot forging process is subjected to the quenching process described later as it is to rapidly cool the aluminum alloy forging, thereby forming a supersaturated solid solution.

In the process performed in conjunction with the temperature rise in the hot forging process, it is preferable to set the temperature immediately after the hot forging process to 510° C. to 560° C. and the time immediately after the hot forging process to the quenching process to 1 second to 30 seconds. Under such a condition, it is possible to achieve a better balance between the costs and the characteristics in the process in conjunction with the temperature rise, as in the case of the solution treatment process.

By performing the solution heat treatment process using the temperature rise in the hot forging process as described above, as compared with the case in which the forging is slowly cooled once after the conventional hot forging process and then reheated in a continuous heating furnace or a single furnace to perform a solution heat treatment process, an aluminum alloy of the same quality can be obtained, the energy required for reheating can be saved, and the production time can also be significantly improved.

Furthermore, an aluminum alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking can be provided at a low cost.

Next, the quenching process, which is the feature of the present invention, is a heat treatment in which a solid solution state obtained in the solution heat treatment process is rapidly cooled to form a supersaturated solid solution.

In this quenching process, the quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to an automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water.

Here, bringing the lower surface into contact with water after the upper surface means that the lower surface is brought into contact with the water after a part of the upper surface is brought into contact with the water. This includes bringing the entire area of the lower surface into contact with the water after the entire area of the upper surface is brought into contact with the water.

FIG. 2 is a diagram schematically showing a quenching process in which the forging 20 is immersed in water at an inclination angle θ with respect to the water surface W when immerging the forging 20 in the water.

Here, the angle θ is defined by the angle formed by the horizontal plane H of the forging 20 and the water surface W. Further, explaining by using the forged material 20 of the shape shown in FIG. 5 as an example, the horizontal plane H of the forging 20 denotes a plane including the centers C1 and C2 of the two assembly holes P1 and P2 of the forging 20. The center C1 is defined as the center of the assembly hole P1 in the thickness direction and the position of the center of gravity of the assembly hole P1 in the plan view. The center C2 is similarly determined.

In FIG. 5, as the forging 20, a shape having two assembly holes P1 and P2 is shown, but the number of assembly holes may be three or more.

In the quenching process of this embodiment, the forging 20 is immersed in water in a state in which the angle θ in FIG. 2 is equal to or less than 10°. With this, the lower surface 21 of the forging 20 is brought into contact with water after the upper surface 22 is brought into contact with the water to perform the quenching process.

Further, as a means for immersing the forging 20 in water in a state in which the angle of the forging 20 with respect to the water surface W is 10° or less, a robot may be used. In this case, the forging 20 is put into water with the forging being grabbed by the robot. Alternatively, a cage may be used. In this case, the cage in which the forging 20 is placed is immersed in water. Furthermore, the present invention is not limited to these means, and any means may be used as long as it is possible to dip the forging into the water so that the angle with respect to the water surface W is 10° or less.

As described above, the quenching process of this embodiment is performed in a state in which the angle θ is 10° or less. However, of course, as shown in FIG. 3, the forging 20 may be horizontally put into the water with respect to the water surface W to perform the quenching process.

Note that in the quenching process of the present invention, in some cases, the forging 20 might be warped. However, in that case, it is enough to correct the warp. Therefore, as shown in FIG. 2 or 3, the lower surface 21 is brought into contact with water after the upper surface 22 is brought into contact with the water to perform the quenching process.

It is preferable that the quenching process be performed by quickly cooling (water quenching) with water at 10° C. to 90° C., especially preferable at 40° C. to 90° C.

As described above, by quenching with water at 40° C. to 90° C., it is possible to provide an aluminum alloy forging for an automobile suspension having high strength and capable of suppressing occurrence of stress corrosion cracking.

As described above, in the quenching process, which is a characteristic of the present invention, the quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to the automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water. Therefore, the lower surface of the aluminum alloy forging is less likely to be rapidly cooled as compared with the upper surface, so that the lower surface has a lower strength than the upper surface. As a result, it is possible to prevent the excess supersaturated solid solution from being retained. As described above, since the lower surface does not retain the excessive supersaturated solid solution as compared with the upper surface, excessive strength is not obtained even if the artificial age hardening process described later is performed.

The artificial age hardening process is a heat treatment for heating and holding the aluminum alloy forging at a relatively low temperature to precipitate a supersaturated solid solution element to impart the appropriate hardness.

In this embodiment, it is preferable to set the heating temperature to 160° C. to 250° C., and the retention time at 10 minutes to 8 hours. This is because the balance between the costs and the characteristics is improved by setting such condition.

In this embodiment, by performing the above-described heat treatments (solution heat treatment process, quenching process, and artificial age hardening process), it is possible to obtain an aluminum alloy forging which is uniformly dispersed in fine precipitates and highly balanced in strength, ductility, and toughness.

FIG. 5 is a perspective view showing an aluminum alloy forged product 20 for an automobile suspension obtained by the production method of the present invention. When assembled to the automobile, the forged product 20 spreads in a direction perpendicular to the thickness T direction rather than the thickness T direction, and has a shape in which there is no large protrusion in the thickness T direction. This is because it is difficult to produce the forged product in the hot forging process if there are protruding parts.

As described above, in the production method of an aluminum alloy forging for an automobile suspension, the quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to the automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water. Therefore, the lower surface of the aluminum alloy forging is less likely to be rapidly cooled as compared with the upper surface, so that the lower surface has a lower strength than the upper surface, thereby reducing the notch sensitivity of the lower surface. Therefore, it is possible to provide an aluminum alloy forging for an automobile suspension capable of suppressing occurrence of stress corrosion cracking.

Further, by using the forged product 20 obtained by the production method of the present invention, when the automobile actually travels on the road surface, damage is received from external factors (wastewater or a snow melting agent, etc.) from the road surface, but the surface with which the external factors come into contact is the lower surface quenched by contacting water later, that is, the surface which does not have excessive strength and has low notch sensitivity. Therefore, it is possible to suppress occurrence of stress corrosion cracking even if corrosion occurs.

Further, in the forged product 20, since the lower surface is brought into contact with water after the upper surface is brought into contact with the water to perform the quenching process, the lower surface requires more time to cool to a predetermined temperature than the upper surface. Therefore, from Table 3 of Non-Patent Document 1 (described on page 751 of Non-Patent Document 1), it can be understood that the lower surface has a longer life by stress corrosion cracking than the upper surface.

The aluminum alloy forged product produced in this way has excellent tensile properties at room temperature. In addition, the surface susceptible to external factors is less sensitive to stress corrosion cracking. Therefore, for example, it is suitably used as a material for an automobile suspension component (suspension arm, upper arm, lower arm, tie rod end, etc.)

INDUSTRIAL APPLICABILITY

The aluminum alloy forged product for an automobile suspension obtained by the production method of the present invention has low sensitivity to stress corrosion cracking in the surface susceptible to disturbance, so it is suitably used as a material, such as, e.g., a suspension arm, an upper arm, a lower arm, and a tie rod end for an automobile, but not particularly limited to such an application.

This application claims priority to Japanese Patent Application No. 2019-209667, filed on Nov. 20, 2019, the disclosure of which is incorporated herein by reference in its entirety.

The terms and expressions used herein are for illustration purposes only and are not used for limited interpretation, do not exclude any equivalents of the features shown and stated herein, and it should be recognized that the present invention allows various modifications within the scope of the present invention as claimed.

DESCRIPTION OF SYMBOLS

-   10: Casting (cast material) -   20: Forging (forged material) -   21: Bottom surface -   22: Upper surface 

1. A production method of an aluminum alloy forging for an automobile suspension, the method comprising, as heat treatment processes: a solution heat treatment process; a quenching process; and an artificial age hardening process, wherein the quenching process is performed by bringing a lower surface of the aluminum forging to be disposed on a ground side when assembled to an automobile into contact with water after an upper surface of the aluminum forging opposite to the lower surface is brought into contact with the water.
 2. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 1, wherein an aluminum alloy constituting the aluminum alloy forging is an Al—Mg—Si based alloy.
 3. The production method of aluminum alloy forging for an automobile suspension as recited in claim 1, further comprising: a hot forging process performed immediately before the solution heat treatment process, wherein the solution heat treatment process is performed in conjunction with a temperature rise in the hot forging process.
 4. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 2, further comprising: a hot forging process performed immediately before the solution heat treatment process, wherein the solution heat treatment process is performed in conjunction with a temperature rise in the hot forging process.
 5. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 1, wherein a temperature of the water in the quenching process is 40° C. to 90° C.
 6. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 2, wherein a temperature of the water in the quenching process is 40° C. to 90° C.
 7. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 3, wherein a temperature of the water in the quenching process is 40° C. to 90° C.
 8. The production method of an aluminum alloy forging for an automobile suspension as recited in claim 4, wherein a temperature of the water in the quenching process is 40° C. to 90° C. 